The hack is the latest setback for Facebook during a year of tumult for the global social media service.

In a blog post, the company says hackers exploited its “View As” feature, which lets people see what their profiles look like to someone else. Facebook says it has taken steps to fix the security problem and alerted law enforcement.

To deal with the issue, Facebook reset some logins, so 90 million people have been logged out and will have to log in again. That includes anyone who has been subject to a “View As” lookup in the past year.

Facebook says it doesn’t know who is behind the attacks or where they’re based.

News broke early this year that data analytics firm that once worked for the Trump campaign, Cambridge Analytica, had gained access to personal data from millions of user profiles. Then a congressional investigation found that agents from Russia and other countries have been posting fake political ads since at least 2016. Facebook CEO Mark Zuckerberg appeared at a Congressional hearing over Facebook’s privacy policies in April.

In a call with reporters on Friday, Zuckerberg said that the company doesn’t know yet if any of the accounts that were hacked were misused.

Facebook has more than 2 billion users worldwide. The company said people do not need to change their Facebook passwords, but anyone having trouble logging on should visit the site’s help center. Those who want to log out can visit the “Security and Login” section of their settings, which lists the places that people are logged into Facebook. It has a one-click option of logging out of all locations.

Ed Mierzwinski, the senior director of consumer advocacy group U.S. PIRG, said the breach was “very troubling.”

“It’s yet another warning that Congress must not enact any national data security or data breach legislation that weakens current state privacy laws, preempts the rights of states to pass new laws that protect their consumers better, or denies their attorneys general rights to investigate violations of or enforce those laws,” he said in a statement.